The invention relates to a device for converting small currents impressed at the input of the device by a non-ideal source of current, into voltage signals, comprising a converter for transforming the small currents into voltage signals.
Devices for converting small current signals into voltage signals are known from practical application, for example from Hewlett Packard: xe2x80x9cOptoelectronics Designer""s Catalogxe2x80x9d, U.S.A. 5091-4573E (6/93) pages 5-32; 5-36; 566; 5-123. This document shows various circuits by which current signals which are provided by a source of current comprising a photo diode can be converted into voltage signals for further application.
However, with the currents provided by sources of signal current with non-ideal components it is not possible to generate voltages which meet the requirements of certain applications by a current-voltage conversion without further treatment. In addition the currents of any circuit elements that may additionally be used have a disturbing influence on the voltage signal issued.
It is the object of the invention to provide a device with which conversion into voltage signals of small current signals, generated by a source of current with non-ideal components in which the non-ideal components of the source of current to a large extent have no effect on the useful signal, and at the same time any disturbing influence of further currents used for controlling the circuit is minimized.
According to the invention, with a device of the type mentioned in the introduction, this object is met in that the output of the converter is fed back to the input of the converter by means of at least one feedback branch and a coupler, with the coupler being switched in such a way that the control currents, provided by the feedback branch of which there is at least one, to control the small currents impressed by the non-ideal source of current at the input of the device, can be coupled in at the input of the converter by means of the coupler with reduced capacitive, inductive and ohmic influence of the feedback branches as well as adjusted amplitudes of the control currents.
One advantage of the device according to the invention is that by feeding back the output signal of the device to the input by way of the coupler, the influence of the non-ideal components of the source of current to the signal current can be compensated. Where present, the currents of the feedback branches are switched directly to the input by coupling. In this, the coupler is configured in such a way that the current emanating from the source of current can be applied to the input of the device controlled by the control currents of-the feedback branches. As a result of the amplification function of the coupler with an amplification factor smaller than 1, in particular large control currents, easier handling and larger tolerances in the components used become possible.
In a preferred embodiment, one of the feedback branches of the device comprises an amplification back-coupler for setting the amplification of the circuit and for compensating for the effects of non-ideal components of the source of current in the input current. With such an amplification back-coupler, parasitic influence factors of the feedback branch can be minimized. Likewise, by the amplification back-coupler providing a compensation current, the voltage swing at the input of the device can be minimized.
Also of advantage is an embodiment in which an operating-point setting device/temperature drift compensator is provided in one of the feedback branches, which makes it possible to adjust the operating point across the entire temperature range. In this way it is possible not only to provide once-only adjustment of a static operating point, which is irrespective of the useful signal, but also to compensate in a continuous manner for the drift in the work point caused by temperature changes of the components used.
A further advantageous embodiment of the device according to the invention has a signal swing limiter in one of the feedback branches, for reducing the amplification of the device and for limiting the signal swing at the input of the device. In this, the threshold for applying the effect of the signal swing limiter is determined in such a way that is commences later than the amplification back-coupler. By limiting the signal swing, overloading the device is avoided, and by limiting amplification, the effects of the non-ideal components of the signal current source are minimized.
With a few additions, the device according to the invention can be used to particular advantage for xe2x80x9c10 Base Txe2x80x9d, xe2x80x9c100 Base Txe2x80x9d or gigabase applications.
Apart from the additions which are necessary to obtain the differential current signal required for xe2x80x9c10 Base Txe2x80x9d applications, an adaptation for xe2x80x9c10 Base Txe2x80x9d applications may also provide as an interface a connecting unit which enables simple changing between PC and network distributor by changing the plug position. In addition, by a respective switching of the pins in the connecting unit, the necessity for an additional supply connection can be avoided if the supply voltage is provided by way of the plug-connection of the device according to the invention which is not used for the differential current signals for the xe2x80x9c10 Base Txe2x80x9d application.
But the use for other applications too, such as profibus, interbus etc. is possible by means of additions corresponding to the requirements of the particular application.
Further advantageous embodiments of the invention are disclosed in the subordinate claims.